A method for obtaining extremely wide-band light modulation by traveling-wave interaction in electro-optic or magneto-optic materials is described. When the intrinsic velocity of microwaves in the material is appreciably less than that of light, the synchronism conditions require the light to be directed obliquely with respect to the wave vector of the microwave field. Structures are described which satisfy this condition, and in which the microwave field is largely concentrated in the electro-optic medium, resulting in very little dispersion and economical use of microwave power. The use of the linear electro-optic effect in potassium dihydrogen phosphate (KDP) in such applications is considered.